The present invention relates to a catalyst for the decomposition of an organohalogen compound, and more particularly, to a catalyst for the decomposition of an organohalogen compound for efficiently decomposing aromatic organohalogen compounds such as dioxins and precursors thereof, which are contained in a very small amount in an exhaust gas discharged from waste incinerators, or aliphatic organohalogen compounds such as trichloroethylene and dichlorometahne.
Exhaust gases discharged from incinerators for incinerating municipal solid wastes or industrial wastes contain a small amount of aromatic chlorine compounds called "dioxins" which have an extremely strong toxicity to human bodies. The dioxins are a generic name of compounds formed by substituting hydrogen atoms of dibenzo-p-dioxine, dibenzofuran or the like with chlorine atoms.
Further, aliphatic organohalogen compounds such as trichloroethylene or tetrachloroethylene have been widely used in various applications such as degreasing of metal or dry-cleaning.
These organohalogen compounds are not only difficult to decompose, but also have a carcinogenic property. Therefore, there arises such a problem that the disposal of these compounds causes environmental pollution by diffusion in air and dissolving-out into ground water or soils. There have been proposed various methods for removing these organohalogen compounds. However, any of the conventional methods has failed to establish economical and efficient techniques for decomposing the organohalogen compounds and converting the compounds into unharmful ones.
Conventionally, various techniques for removal and decomposition of the organohalogen compounds have been reported. For example, there are known a method of decomposing poly-halogenated aromatic compounds having at least five carbon atoms by heating at a temperature of 200 to 550.degree. C. in the presence of a catalyst such as iron oxide (Japanese Patent Publication (KOKOKU) No. 6-38863(1994)); a method of removing halogenated aromatic compounds or the like from an exhaust gas or reducing amounts thereof by heat-treating at a temperature of 300 to 700.degree. C. in the presence of a catalyst containing iron oxide (Japanese Patent Application Laid-Open (KOKAI) No. 2-280816(1990)); a method of introducing an inhibitor containing an activated carbon in which an amine compound is carried thereon, for inhibiting the generation of dioxins, into exhaust gas flues or the like of an incinerator (Japanese Patent Application Laid-Open (KOKAI) No. 11-9960(1999)); a method of decomposing an organohalogen compound at a temperature of from 60.degree. C. to less than 150.degree. C. in the presence of oxygen using a iron oxide-based and/or titanium dioxide-based solid catalyst (Japanese Patent Application Laid-Open (KOKAI) No. 11-188235(1999)); or the like.
In addition, as the methods comprising preliminarily mixing wastes with iron oxide or the like and then incinerating the wastes, there are known a method of burning combustible wastes at a temperature of not less than 850.degree. C. under the coexistence of an acid gas neutralizing agent, iron oxide particles and the like (Japanese Patent Application Laid-Open (KOKAI) No. 8-270924(1996)); and a method comprising burning wastes in an incinerator under the coexistence of iron oxide hydroxide particles or iron oxide particles containing sulfur and sodium in less than predetermined amounts (Japanese Patent Application Laid-Open (KOKAI) No. 9-89228(1997)).
Although it has been desired to provide a method for treating an exhaust gas so as to decompose and remove organohalogen compounds contained therein, the methods described in the above publications are still unsatisfactory.
Namely, in the method described in Japanese Patent Publication (KOKOKU) No. 6-38863(1994), poly-halogenated cycloalkyl compounds and poly-halogenated aromatic compounds in fly ash generated in an incinerator are decomposed by catalysts such as iron oxide, calcium carbonate and sodium carbonate in a fixed bed. However, in this method, it is difficult to sufficiently remove the organohalogen compounds for a short period of time, and huge plant and equipment investment is required to construct a facility for converting the fly ash into unharmful substances. Such a construction is almost impossible practically.
In the method described in Japanese Patent Application Laid-Open (KOKAI) No. 2-280816(1990), after ammonia is added to an exhaust gas containing halogenated aromatic compounds, the halogenated aromatic compounds are decomposed in the presence of an iron oxide-containing catalyst in a fixed bed. Therefore, the construction of such a complicated facility after the waste incinerator also requires huge plant and equipment investment.
In the method described in Japanese Patent Application Laid-Open (KOKAI) No. 11-9960(1999), the amine-carrying activated carbon is introduced into an exhaust gas containing dioxins, so that the dioxins are adsorbed into the activated carbon by the adsorptivity thereof, and then decomposed by reacting with the amine compound. The amine-carrying activated carbon has a high adsorptivity, but is incapable of sufficiently decomposing the dioxins and inhibiting the generation of dioxins. Further, the activated carbon has a risk of ignition at an elevated temperature, thereby causing problems concerning safety.
In the method described in Japanese Patent Application Laid-Open (KOKAI) Nos. 11-188235(1999), the organohalogen compounds contained in the exhaust gas are decomposed at a temperature of from 60.degree. C. to less than 150.degree. C. in the presence of oxygen using the iron oxide-based and/or titanium dioxide-based solid catalysts. In this method, although the organohalogen compounds are decomposed at a relatively low temperature, the decomposition percentage (conversion rate) is low and, therefore, impractical.
In the methods described in Japanese Patent Application Laid-Open (KOKAI) Nos. 8-270924(1996) and 9-89228(1997), it is required to sufficiently premix solid wastes with iron oxide particles. Therefore, it is not easy to conduct this method.
Meanwhile, in the incineration method using iron oxide hydroxide particles or iron oxide particles containing sulfur and sodium in not more than predetermined amounts (Japanese Patent Application Laid-Open (KOKAI) No. 9-89228(1997)), the organohalogen compounds such as dioxins in exhaust gases cannot be sufficiently decomposed due to the low monochlorobenzene decomposition percentage thereof at 300.degree. C. as described in Table 1 (Iron Compound 4) hereinafter.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that by contacting a combustion exhaust gas containing organohalogen compounds with a composite particles as an organohalogen compound-decomposition catalyst, which comprise iron compound particles having an average particle size of 0.01 to 2.0 .mu.m, a phosphorus content of not more than 0.02% by weight, a sulfur content of not more than 0.3% by weight and a sodium content of not more than 0.3% by weight, and an amine compound, the organohalogen compounds can be decomposed at a high efficiency. The present invention has been attained on the basis of this finding.